Hitherto, there are proposed and used disc drive apparatuses using, as recording medium, various optical discs such as CD (Compact Disc), DVD (Digital Versatile Disc), and/or BD (Blu-ray Disc), etc. etc.
Meanwhile, light beams used for reproducing CD, light beams used for performing recording of information onto DVD or performing reproduction of recorded information, and light beams used for performing recording of information onto BD or performing reproduction of recorded information have wavelengths different from each other. For example, the wavelength of light beams used for reproduction of CD is substantially 780 mm, the wavelength of light beams used for recording or reproduction of DVD is 650˜660 nm, and the wavelength of light beams used for recording or reproduction of BD is substantially 405 nm.
Further, object lenses provided at optical pick-up devices (hereinafter referred to as optical pick-up units as occasion may demand) used for converging light beams onto CD, DVD and BD to perform recording or reproduction of information have numerical apertures (NA) different from each other.
Furthermore, in respective optical discs of CD, DVD and BD, thicknesses of disc bases (substrates) on which information recording layer is provided are different from each other.
There are provided disc drive apparatuses capable of selectively using optical discs different in the recording format, e.g., CD and DVD in which light beams different in wavelength are used and object lenses different in numerical aperture (NA) are further used as described above so that recording and/or reproduction of information are performed.
Further, there are proposed disc drive apparatuses capable of selectively using three kinds of optical discs of CD, DVD and BD which are different in the recording format.
In disc drive apparatuses of this kind, it is conceived to use optical pick-up units adapted for converging, by using single object lense, light beams having wavelengths different from each other, which are used in recording or reproduction of CD, DVD and BD, to irradiate the light beams thus converged onto the loaded optical disc. The optical pick-up units adapted for selectively converging, onto an optical disc, light beams different in wavelength by using single object lens as stated above have the problems that not only the object lens becomes large-sized so that enlargement of the unit itself cannot be avoided, but also manufacturing cost is increased.
In order to realize elimination of such problems, as optical pick-up units used in disc drive apparatuses adapted for selectively using three kinds of optical discs of CD, DVD and BD which are different in the recording format, optical pick-up units using two object lens are proposed and are provided in practice. Optical pick-up units of this kind each comprises an object lens for converging light beams for CD and DVD, and an object lens for converging light beams for BD.
It should be noted that, as optical pick-up units in which two object lenses are provided to respectively converge, by these object lenses, two kinds of light beams different in wavelength to irradiate the light beams thus converged onto an optical disc, there are optical pick-up units described in the Japanese Patent Application Laid Open No. 1999-23960 publication, the Japanese Patent Application Laid Open No. 1999-259891 publication and the Japanese Patent Application Laid Open No. 1999-64724 publication.
Meanwhile, optical pick-up units adapted to perform recording or reproduction with respect to three kinds of optical discs of CD, DVD and BD which are different in the recording format and adapted to converge three kinds of light beams which are different in wavelength by using two object lenses are required to have more rigorous accuracy for correction of aberration such as spherical aberration or comatic aberration, etc. as compared to optical pick-up units adapted to converge two kinds of light beams different in wavelength by using two object lenses.
Namely, in optical pick-up units in which two object lenses are provided so as to comply with two kinds of optical discs of CD and DVD to respectively converge two kinds of light beams different in wavelength by these object lenses, with respect to correction of aberration such as spherical aberration and/or comatic aberration, etc., adjustment of the optical system is performed at accuracy required for recording or reproduction of DVD so that the condition of correction accuracies of respective aberrations required for CD can be satisfied.
However, in optical pick-up units adapted for converging three kinds of light beams different in wavelength by using two object lenses in order to comply with three kinds of optical discs of CD, DVD and BD which are different in the recording format, it is required to correct aberration such as spherical aberration and/or comatic aberration, etc. taking place when object lens used in CD and DVD is used so that it falls within the range of accuracy required for DVD, and it is further required to correct aberration such as spherical aberration and/or comatic aberration, etc. taking place when object lens used in BD is used so that it falls within the range of accuracy required for BD.
In optical pick-up units adapted to comply with three kinds of optical discs of this kind, it is required for both two object lenses that aberration correction of high accuracy is performed.
Here, an example of a method of correcting aberration such as spherical aberration and/or comatic aberration, etc. taking place by light beams emitted from the optical pick-up unit comprising single object lens and irradiated onto an optical disc will be explained.
In the optical pick-up units of this kind, there are provided a comatic aberration correcting device for suppressing occurrence of aberration such as spherical aberration and/or comatic aberration, etc. taking place when light beams as described above are converged onto the signal recording surface of the optical disc, and a triaxial actuator serving as an object lens drive unit for allowing the object lens to undergo drive displacement. The triaxial actuator serves to allow an object lens for converging light beams onto the signal recording surface of the optical disc to undergo drive displacement, and is operative to allow the object lens to undergo drive displacement in three axes directions of a focus direction which is a direction in parallel to the optical axis of the object lens, a tracking direction which is a direction traversing recording tracks provided at the optical disc onto which light beams are irradiated and is a radial direction which is a direction perpendicular to the focus direction, and a radial tilt direction which is a rotational direction in which tangential direction of recording tracks is caused to be center.
The state where triaxial actuator as described above is used to allow the object lens to undergo drive displacement will be explained in more concrete terms.
As shown in FIG. 1, the optical pick-up unit comprising triaxial actuator is disposed within a disc drive apparatus in which a spindle motor 2 for performing rotational operation of an optical disc 1 is included.
The triaxial actuator is disposed in a manner facing the optical disc 1 which is caused to undergo rotational operation by the spindle motor 2, and is adapted so that an object lens 4 is mounted on a bobbin 3 caused to undergo drive displacement by the actuator.
Although not shown, light beams L emitted from light source such as semiconductor laser, etc. are converged by the object lens 4, and are irradiated onto the signal recording surface of the optical disc 1.
In the optical pick-up unit shown in FIG. 1, the object lens 4 mounted on the bobbin 3 is caused to undergo drive displacement by the triaxial actuator in three axes directions of focus direction F which is a direction in parallel to the optical axis of the object lens 4, tracking direction T which is a direction traversing recording tracks provided at the optical disc 1 onto which light beams L are irradiated and is a direction perpendicular to the focus direction F, and radial tilt direction Rad/Tilt which is a rotational direction in which tangential direction Tan which is a tangential direction of recording tracks is caused to be center.
It is to be noted that the radial tilt direction Rad/Tilt is a direction in which movement is performed in a circular arc form on the axis of the tracking direction T with the point A located below by a predetermined distance relative to the bobbin 3 being as center in FIG. 1. In addition, the direction in which movement is performed in a circular arc form on the axis of the tangential direction Tan with the point A being as center refers to the tangential tilt direction Tan/Tilt.
The triaxial actuator performs drive displacement of the bobbin 3 on the basis of a focus error signal obtained by detecting whether or not light beams L irradiated through the object lens 4 is in in-focus state on the signal recording surface of the optical disc 1, a tracking error signal obtained by detecting whether or not light beams L are positioned on recording tracks of the optical disc 1, and a radial tilt control signal obtained by detecting tilt quantity with respect to the optical disc 1 of light beams L so that the object lens 4 is caused to undergo drive displacement in the focus direction F, in the tracking direction T and in the radial tilt direction Rad/Tilt.
As the result of the fact that the object lens 4 is caused to undergo drive displacement in accordance with the above-described error signals of three axes directions, light beams L emitted from the object lens 4 are caused to be placed in in-focus state onto the signal recording surface of the optical disc 1, and are positioned on a predetermined recording track. Further, those light beams L are vertically incident on the signal recording surface of the optical disc 1 without being tilted thereto. As a result, it is possible to precisely scan recording tracks of the optical disc 1 by light beams L. Thus, it is possible to perform recording or reproduction with high accuracy.